This invention relates to a polarization independent optical isolator.
Conventional polarization independent optical isolators which are believed to be relevant to this invention have been hitherto proposed in, for example, Japanese Pat. Publication No. SHO 58-28561(B) (corresp. to U.S. Pat. No. 4,239,329) and Japanese Pat. Publication No. SHO 60-49297(B) (corresp. to U.S. Pat. No. 4,178,073).
One example of the prior art optical isolator noted above will be described hereinafter with reference to the schematic diagram of FIG. 1. As illustrated in FIG. 1(A), the conventional optical isolator comprises two birefringent crystals 1, 2 having the same thickness, one magneto-optic element 3 providing 45.degree. of Faraday rotation, and an optically active crystal 4 whose optical axis and thickness are so determined that the direction of polarization of the light transmitting through the optically active crystal is shifted by 45.degree. relative to that of the light incident thereon. The optical isolator is disposed between two optical fibers 5 and 6.
The light emitted from the optical fiber 5 passes through the first birefringent crystal 1 while separating polarized component thereof into ordinary and extraordinary rays. Subsequently, the two polarized components thus separated transmit through the magneto-optic element 3 to thereby rotate the polarizations thereof by 45.degree. in the counterclockwise direction as viewed from the optical fiber 6. The polarizations of these polarized components are further rotated by 45.degree. while passing through the optically active crystal 4. Thereafter, the polarized components in the separate state pass through the second birefringent crystal 2 to be synthesized. The synthesized light finally enters into the optical fiber 6.
The light having two polarized components noted above reflectively emanates from the optical fiber 6 in a reverse direction and transmits through the second birefringent crystal 2 and the optically active crystal 4 while being subjected to polarization rotation in the same direction as that in which the forward-directed light advances. Nevertheless, the polarized components traveling in the reverse direction undergo 45.degree. counterclockwise rotation of polarization as viewed from the optical fiber 6 by the magneto-optic element 3. As a result, the polarized components emanating from the magneto-optic element 3 are shifted by 90.degree. relative to the forward-directed polarized components. Besides, the polarized components traveling in the reverse direction which have been subjected to polarization rotation are further separated by the first birefringent crystal 1 and enter into the optical fiber 5 along a path out of the light axis of the forward-directed light.
The changes in direction of polarization, which are effected in the light traveling from the optical fiber 5 to the optical fiber 6 at the positions P1, P2, P3, P4 and P5 in the forward direction, will be explained hereinbelow with reference to FIG. 1(B).
In the drawing, there are indicated a vertically polarized component F1 of the forward-directed light when entering into the first birefringent crystal 1 by the dotted line, a horizontally polarized component F2 by the solid line, the centers C1 and C2 of the polarized components by the black spots, and the center axis O of the incident light propagating forwardly by the circle.
The vertically polarized component F1 of the forward-directed light emitted from the optical fiber 5 and passing across the first birefringent crystal 1 advances straight as the ordinary ray. On the other hand, the horizontally polarized component F2 propagating as the extraordinary ray is shifted laterally by the first birefringent crystal 1. Thus, the center C2 of the polarized component F2 is deviated from the center C1 of the polarized component F1 as shown at the position P2.
The polarized components F1, F2 emanating from the first birefringent crystal 1 undergo 45.degree. counterclockwise rotation as viewed from the optical fiber 6 when passing through the magneto-optic element 3. As a result, the polarized components F1, F2 are inclined respectively by 45.degree. in opposite directions, as shown at the position P3.
The polarizations of these polarized components entering into the optically active crystal 4 are further rotated by 45.degree. in opposite directions at the position P4. Consequently, the polarized components respectively become at right angles to those incident upon the first birefringent crystal 1 at the position P1.
Subsequently when the polarized components emanating from the optically active crystal 4 pass through the second birefringent crystal 2, the polarized component F1 is shifted in parallel and the component F2 advances straight, with the result that the centers C1 and C2 of the polarized components F1 and F2 coincide with each other at the position P5.
The polarized components F1a and F2a traveling in the reverse direction propagate, as shown in FIG. 1(C), from the position P5 to the position P3 via the position P4 in the same manner as the forward-directed polarized components F1 and F2, whereas these polarized components are subjected to 45.degree. of polarization rotation by the magneto-optic element 3 as shown at the position P2. Thereafter, the centers C1a and C2a of the polarized components F1a and F2a are deviated from the center axis O at the position P1.
Since the center C1a of the polarized component F1a agrees with the center axis O at the positions P4, P3 and P2, the reverse-directed polarized components travel backward to the first birefringent crystal 1 along the center axis O of the forward-directed polarized components.
Accordingly, the performance of the conventional optical isolator is very dependent on the performance of the first birefringent crystal 1 and the mechanical accuracies of the birefringent crystals and magneto-optic element (particularly, the accuracy of the plane of polarization of the first birefringent crystal). In the optical isolator illustrated in FIG. 1, in order to heighten the effect of preventing the reverse-directed light from entering into the optical fiber 5 disposed on the light source side, it can be expected that a plurality of optical units each constituted by the two birefringent crystals 1, 2, one magneto-optic element 3 and one optically active crystal 4 as noted above are used in an optical system. Whereas, such an optical system would entail a disadvantage that it cannot be miniaturized and inevitably turns out to be expensive because the number of component elements is increased, though the desired effect may be somewhat improved according to the increase in number of optical units. Besides, since the returning light (reverse-directed light) travels along the center of the forward-directed light to the first birefringent crystal, the efficiency of propagating the light in the optical system is apt to be deteriorated.
One object of this invention is to provide an optical isolator having a structure capable of decreasing the number of constituent elements and being made compact. Another object of this invention is to provide an optical isolator having excellent efficiency and performance which are independent on mechanical accuracy of the component elements constituting an optical system. Still another object of this invention is to provide an excellent optical isolator capable of be manufactured at a low price.